1. Field of the Invention
The present invention relates to a lateral electric field liquid crystal display device. More specifically, the present invention relates to an electrode structure of an active element substrate.
2. Description of the Related Art
In general, a lateral electric field liquid crystal display device is constructed such that a pixel electrode and a common electrode are provided on the same, substrate, and an electric field is applied therebetween. Each pixel electrode is connected to an active element, such as a thin-film transistor (TFT), to constitute an active matrix substrate (TFT substrate). Liquid crystal is filled between the TFT substrate and an opposing substrate. In this lateral electric field liquid crystal display device, since the liquid crystal is controlled by the electric field applied parallel to a substrate surface, a wide viewing angle can be obtained.
The TFT substrate of the type described above will be explained with reference to FIGS. 1 and 2. FIG. 1 is a plan view of one pixel of the TFT substrate of the conventional liquid crystal display device as viewed from the side of liquid crystal. FIG. 2 is a cross-sectional view taken along the line X-X′ of FIG. 1.
A scanning line 103, a gate electrode 102 connected to the scanning line 103, a common line 104, and a common electrode 105 extending as a part of the common line 104 are formed of a metal film in the same layer on a transparent insulation substrate 101 formed of, for example, glass. A gate insulating film 106 is formed over these lines and electrodes. The common electrode 105 is partially formed parallel to a pixel electrode described below. Subsequently, a semiconductor island 107 is formed on the gate insulating film 106. Thereafter, a signal line 110, source/drain electrodes 108 and 109 of a TFT, both of which are partially formed on the semiconductor island 107 to be connected thereto, and a pixel electrode 111 connected to the source/drain electrode 109 are formed using the same metal film as that used to form the source/drain electrode. Note that both the pixel electrode 111 and the previously formed common electrode 105 are partially formed in parallel with each other. In addition, an interlayer insulating film 112 and an alignment layer 113 are formed in such a manner as to cover the semiconductor island 107, the source/drain electrodes 108 and 109, and the pixel electrode 111. In this manner, formation of TFT substrate 300 is completed. Note that a storage capacitor is formed where the pixel electrode 111 and the common line 104 overlap each other (an overlap portion 122 indicated as a cross-hatched portion).
Light shielding film 152 is formed on the transparent insulation substrate 151 to partition pixels on the TFT substrate 300 and cover the semiconductor island 107. An insulating film 153 and an alignment layer 154 are formed to cover the light shielding films 152, thereby constituting an opposing substrate 400.
Liquid crystal 127 is filled between the TFT substrate 300 and the opposing substrate 400 thus to form a liquid crystal display device. In FIG. 2, a polarizer and the like are not shown for simplification.
It should be note that the scanning line 103, the common line 104 and the signal line 110 are formed to have enough thickness to reduce resistance values thereof. Since the common line 104 and the common electrode 105 are formed together with the scanning line 103, all of which have the same thickness. Consequently, as shown in FIG. 2, the interlayer insulating film 112 covering these electrodes and lines is formed with large variation in a height of step. In this case, when an alignment layer material coated having large height of step on the interlayer insulating film 112 is subjected to a rubbing process, the material cannot be rubbed uniformly over the TFT substrate. As a consequence, the alignment layer 113 is formed unevenly due to nonuniformity of rubbing, thereby making the liquid crystal display device have nonuniformity of display.